A rogue protein injected into the brains of mice led to a cascade of effects that culminated in Parkinson’s-like symptoms in an experiment that may, for the first time, establish the disease’s path of destruction.

Healthy mice injected with the abnormal protein began producing it in their brains, leading to clumps of the substance called Lewy bodies. The clumps in turn caused loss of dopamine- producing neurons that are key to motor control, according to a study in the journal Science.